FOURTH, OR CORONICERAN BRANCH. 177 



more prominent. Upon either the latter part of the fourth, or first quarter of 

 the fifth volution, the whorl attained its adult characteristics. 



During the first volution the increase laterally had been very great, forming 

 a deep umbilicus ; subsequently the tubercles and folds were added to the width 

 of the abdomen of the earlier whorls, giving a general resemblance to Cor. latum. 

 The young whorls of specimens in which the tubercles appeared later on the 

 third whorl were usually rounder, and exhibited, when seen from the side, only a 

 very remote resemblance to Cor. latum. These passed suddenly, so fast did the 

 tubercles grow to be folds and then true pilas, into the stage in which they re- 

 sembled Cor. Jcridion. After this stage, the preponderance of the abdominal 

 region is not so marked, though it may be maintained throughout the fourth 

 volution. Quenstedt's young specimen, with a living chamber one volution in 

 length, 1 exhibited a broad abdomen on the fourth whorl. 



The genicular are at first not tuberculated, but sharp and angular, as in some 

 varieties of Cor. Jcridion. The abdomen, however, does not usually become suffi- 

 ciently prominent in the earlier stages of growth to bear comparison with that 

 region in Jcridion. After this period the umbilical shoulders are developed more 

 proportionally, and finally on the fifth volution the whorl becomes broader dor- 

 sally than near the abdomen. The dorsal curves of the pilae become more 

 prominent, and sink or are contracted to form the tubercular genicular. These, 

 subsequently bend forward in some specimens, ascending the abdomen. This 

 last character may occur earlier, on the third whorl, or, on the other hand, be 

 omitted altogether, even in adults. 



This description of the stages of development may be curiously altered by a 

 malformation of the abdomen. The pilae, Plate III. Fig. 7, ll, 2 are continued 

 across, cutting up the abdomen into a series of waves entirely obliterating the 

 keel and channels. 



This variety has young which differ considerably from one another in the 

 relative breadth of the abdomen, some being excessively broad and flat during 

 the young whorls, and others, especially the microceran forms, have much nar- 

 rower whorls. The number of the pilae also differs, these parts being more 

 widely separated in the broad, flat young than in others. 



In the majority of adult specimens, the superior lateral saddles are deeply 

 divided by two marginal lobes, and spread out laterally beyond, or on the ge- 

 nicular. One specimen, however, from Semur, though not differing in other 

 respects, has superior lateral saddles so long and narrow that the superior lateral 

 lobes occupy the genicular line. The abdominal lobe extends beyond the supe- 

 rior laterals by about one third, and the superior lateral saddles are of about 

 the same depth as the inferior laterals, though much broader. 



Var. B. 

 This variety is founded on two specimens, which maintained a broad ab- 

 domen and the immature, thick, unshapely pilae of the young until a late 



1 Figured in Amm. Schwab. Jura, pi. xv. fig. 9. 



a Fig. 7 represents the abdomen erroneously, the forward projections of the pilte where they meet on 

 the abdomen being flattened out in the specimen. 



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